core: adding CGroup for io.systemd.Unit.List (second PR) (#37646)

This PR adds CGroup context/runtime for io.systemd.Unit.List method.

This is follow up for https://github.com/systemd/systemd/pull/37432.

diff --cc src/core/meson.build
index f1a7521c0bcf14993694d9406dac35603b0ab028,89376426df46a73e5c55f0377b2e09b6c8bf472e..db44e7b80ea1d7cf3b98f7a6404882cd058cbb3c
--- 1/src/core/meson.build
--- 2/src/core/meson.build
+++ b/src/core/meson.build
@@@ -63,8 -63,8 +63,9 @@@ libcore_sources = files
          'unit-serialize.c',
          'unit.c',
          'varlink.c',
+         'varlink-cgroup.c',
          'varlink-common.c',
 +        'varlink-dynamic-user.c',
          'varlink-manager.c',
          'varlink-unit.c',
  )